This invention relates to apparatus for applying to a surface of a support member at least one ribbon-like stream of a coating composition to form a unitary layer on the surface of the support member and processes for fabricating and using the apparatus.
Numerous techniques have been devised to form a layer of a coating composition on a substrate. One of these techniques involves the use of an extrusion die from which the coating composition is extruded onto the substrate. For fabrication of web type, flexible electrophotographic imaging members, the extrusion die must lay down very thin coatings meeting extremely precise, critical tolerances in the single or double digit micrometer ranges. Moreover, a plurality of dies may be needed to lay down up to three extruded coatings conventionally employed for flexible electrophotographic imaging members. The flexible electrophotographic imaging members may also comprise additional coatings applied by non-extrusion coating techniques so that the finished electrophotographic imaging member can contain as many as 5 different coatings. The extrusion die usually comprises spaced walls, each having a surface facing each other. These spaced walls form a narrow, elongated, passageway. Generally a coating composition is supplied by a reservoir to one side of the passageway and the coating composition travels through the passageway to an exit slot on the side of the passageway opposite the reservoir. Dams are provided at opposite ends of the passageway to confine the coating composition within the passageway as the coating travels from the reservoir to the exit slot. The surface of the dams facing the coating composition is generally perpendicular to the exit slot.
Each end dam seals one end of the die. These end dams have a sharp corner positioned at each end of the exit slot. This sharp corner is formed at the intersection of two adjacent sides of the end dam and is important to the proper operation of the coating process. More specifically, the extruded coating material pins to the sharp corner as the material emerges from the exit slot thereby creating a small, relatively flat edge bead for the deposited coated layer. The expression "pin" as employed herein is defined as the attachment of the coating composition to the extrusion die or end dam. Soft Teflon.RTM. type materials have been used for extrusion die end dams. Teflon.RTM. extrusion die end dams have excellent sealing attributes and resistance to the chemicals utilized for fabricating electrophotographic imaging members. However, Teflon.RTM. also cannot be machined and the edge radius cannot be accurately controlled. Teflon.RTM. extrusion die end dams require time to stabilize when coming on line immediately after installation thereby creating unacceptable coated material that must be scrapped. Moreover, with use, the sharp corner of Teflon.RTM. extrusion die end dams deteriorates due to dents and cuts caused by cleaning operations and shimming of the die lip opening during coating runs. Dents and cuts can occur during the cleaning operation due to collisions with thin metal or plastic material which inserted through the die exit slot and slid back along the die passageway. As this corner breaks down, the extruded coating materials loses its ability to pin to the corner leading to edge bead instability which in turn creates a thicker and wider edge bead due to the necking in and movement of the coating material back on the radius of the corner. When sequentially applied layers having these thicker and wider edge beads are deposited over each other the resulting consolidated edge bead can become substantial due to edge bead build up. This edge bead build up causes a ridge to form above the overlapped edge beads as well as in the substrate below the overlapped edge beads when the coated support member is a flexible electrophotographic imaging web which is subsequently rolled for storage, shipment or for further processing. This ridge is undesirable in precision machines and can cause adverse effects such as electrical arcing and coating damage due to contact with closely spaced machine components. Moreover, edge bead build up tends to promote the formation of blisters when the coatings are applied as solutions containing volatile carrier liquids. Unduly thick and wide edge beads also present coating problems for subsequently applied layers and introduce handling and fabricating problems later in the manufacturing process. Also, production line down time is high because the Teflon.RTM. extrusion die end dams must be replaced frequently. Substitution of ordinary brass end dams for Teflon.RTM. extrusion die end dams results in leakage of the coating material from the ends of the extrusion dies. Aluminum end dams form particulates which contaminate the coating composition which in turn forms electrophotographically unacceptable layers. Stainless steel end dams do not seal well and leak. Many plastic end dam materials are dissolved by the solvents employed in the extruded coating compositions. Further, the extruded coating compositions will not pin to many end dam materials thereby causing the formation of unstable edge beads. Thus the characteristics of common extrusion systems utilizing end dams exhibit deficiencies for processes for manufacturing coated articles having precise tolerance requirements.